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    <h1>1111</h1>
    <script src="ModelA.js"></script>
    <script src="ModelB.js"></script>
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    A B C D
    B依赖A和C
    C依赖A和D
    D依赖B和C

    单例模式只适合做简单的依赖关系，复杂的不行
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